The invention relates to a structure including an insulating part in a massive substrate and a method of producing such a structure.
Such structures find applications in the field of micro-electronics for the manufacture of integrated circuits, certain parts of which must be insulated and free from parasitic effects such as parasitic capacitive effects.
The structures also find applications in the field of micro-mechanics for the production of sensors such as accelerometers or pressure sensors, as well as the creation of actuators. In the case of an accelerometer, the insulated part of the structure is, for example, constituted by a seismic mass for measuring accelerations.
For the production of sensors such as pressure sensors or accelerometers, widespread use is made of SOI type structures. These structures comprise a thin surface layer of silicon separated from an underlying layer of silicon by an embedded silicon oxide layer. The thin layer of silicon is locally freed from the underlying layer. It can then constitute, for example, the membrane of a pressure sensor or, when it is suitably formed, the seismic mass of an accelerometer. Reference may be made to this subject in documents (1) and (2), the references for which are given at the end of this description.
The surface layer can also be formed in such a way that an actuator is produced. Actuators produced according to micro-machining methods are described in documents (3) and (4), the references for which are also given at the end of this description.
The embedded oxide layer has the role of supporting the structure formed in the thin surface layer. This aspect is apparent in particular, in document (2) relating to an accelerometer.
However, because of its small thickness, the electrical or mechanical insulation provided by the insulatinq oxide layer can prove to be insufficient.
In particular, parasitic capacitance phenomena can be induced between the deep underlying layer of silicon and the surface layer of silicon.
These phenomena can affect, in a negative way, the measurements of an accelerometer or those of another sensor that operates according to a capacity measurement principle.
Another disadvantage of SOI type structures and of devices produced from these structures is associated with their cost.
The aim of this invention is to provide a structure with an electrically insulating part, the production of which is not expensive and which does not have the limitations mentioned above.
Another aim is also to provide a structure with an insulating part which is nearly not subject to parasitic capacity effects, and can be used for the production of a sensor. An aim of the invention is also to provide a method of producing such a structure.
In order to achieve these aims mentioned above, the invention has the more precise objective of a structure comprising a first part and at least one second part, electrically insulated from the first part, the first and second parts each being formed in the same wafer of material, said first and second parts each having substantially one and the same thickness, extending substantially in one and the same plane, and having at least one mutually adjacent edge, and, the adjacent edges of the first and second parts being separated by a spacing. Conforming to the invention, the structure comprises, in addition, at least one joint of insulating material, arranged in the spacing, to make the first and second parts integral, the joint being in mechanical contact with at least a portion of each wafer and extending substantially over the entire thickness of the wafers.
The joint of insulating material has a double function. The first function is to provide electrical insulation between the first and second parts. It should be noted that the quality of the electrical insulation can be adapted by making use of the width of the spacing, and hence the width of the joint, as well as the material used to form the joint. A second function of the insulating joint is to maintain the cohesion of the structure by making the first and second parts integral. In particular, the joint allows the wafers to be held with their principal faces substantially in one and the same plane.
Although the structure thus obtained can be linked to a suitable support structure, the second insulated part can be sufficiently far from the support or separated from the first part to prevent capacitive effects, if these are not desired.
On the other hand, in other applications, the adjacent edges of the first and second parts can be configured in such a way and arranged sufficiently close that a variable capacity is formed.
The measurement of this capacity and of its variations then allows the determination of any possible displacement between the first and second parts. This aspect, that permits the production of an accelerometer or an actuator, is described in greater detail in a subsequent part of the description.
According to one advantageous aspect, the first and second parts can have complementary shapes. The spacing between the parts is then substantially uniform.
The first and second parts can be simply wafers placed side by side, obtained by cutting a substrate into two parts along a line. However, according to a variant, the second part can be a portion of a wafer of material, cut in the wafer of material forming the first part, in accordance with a specific pattern.
In the text that follows, reference is only made to one single first part and one single second part. It is however possible to design the structure with one first part and a plurality of second parts, mutually insulated and insulated from the first part. In particular, it is possible to cut in the wafers of material forming the first part, a plurality of portions of wafers which then form a plurality of second parts of the structure.
In accordance with the envisaged applications and in particular for the production of sensors such as, for example, an accelerometer or an actuator, the material of the wafer forming respectively the first and second parts of the structure can be chosen to be non-insulating. By non-insulating material, one understands a semi-conductor or conductor material.
By way of example, the material is chosen from among the following semi-conductor materials AsGa, Si, SiC, the following metals Au, W. Ti, Cr, Al, Fe, Ni, Mo and alloys of these metals.
Another objective of the invention is a method of producing a structure comprising a first part and at least one second part, co-planar with and electrically insulated from the first part. The method includes the following steps:
a) cut a substrate wafer in order to define in the wafer, a first and a second part, at least one portion of the substrate, called the holding portion and linking the first and second parts, being preserved at the time of cutting,
b) substitution of an insulating holding joint, linking at least one portion of the first and second parts over substantially their entire thickness, at said holding portion of substrate connecting the first and second parts of substrate.
By substitution of an insulating holding joint, at said holding portion of substrate connecting the first and second parts, one understands, either the putting into place of said joint and the removal of the portion of substrate, or the transformation of the portion of holding substrate into an insulating joint.
The method of the invention can be implemented in accordance with several possibilities.
According to a first possibility, step a) comprises the formation of at least one cut in the substrate wafer, that goes right through and separates for the main part, the first and second parts. Step b) then comprises the filling of the cut with an insulating material forming the holding joint between the first and second parts, and the removal of the portion of the holding substrate linking the first and second parts.
One may consider that the cut that goes through separates, for the main part, the first and second parts of the substrate wafer when it separates these two parts over the whole of their adjacent edge(s), except for one or more portions of substrate that link the first and second parts in order to maintain the cohesion of the structure during the manufacturing process. These portions are named the holding portion.
According to an implementation variant of the method, step a) comprises the formation of at least one cut that does not go right through the wafer substrate, but which separates for the main part the first and second parts, the cut having a bottom that forms the substrate holding portion linking the first and second parts. In this case, step b) comprises the filling of the cut with an insulating material forming the joint between the first and second parts, and the removal of the bottom of the cut.
The cut is considered not to go right through when a cut bottom remains which links the first and second parts.
This cut bottom maintains the cohesion of the structure until the insulating joint is put into place.
When this joint is formed, the bottom is removed, for example by mechanical, chemical or physical thinning down of the substrate wafer.
The two possible ways of implementing the method, described above are not mutually exclusive. In effect, during the etching of the cuts, it is possible to preserve both a cut bottom and holding portions of the substrate linking the first and second parts over the whole thickness of the wafer.
After the removal of the cut bottom, and if necessary the removal of the other parts of holding substrate linking the first and second parts, the first and second parts are electrically insulated.
According to yet another variant of the method of the invention, the cutting of the wafer during step a) includes piercing it with a plurality of adjacent holes, separated by the side walls of the holes. In this case, step b) comprises the oxidation of the side walls, and, if the need arises, the bottom of the holes in order to convert them into an insulating holding joint.
In the implementation of the invention conforming to this variant, the portions of holding substrate which initially link the first and second parts after the cutting of the substrate wafer comprise the side walls separating the holes. The holding portions of substrate can equally well comprise the bottom of the holes if these are not pierced right through. These parts are not removed in the same way as the substrate portions in the implementations previously described, but are converted, by oxidation, in order to form the insulating holding joint.
Other characteristics and advantages of the invention will more clearly emerge from the description that follows, and that makes reference to the Figures in the appended drawings, which are given purely for illustrative purposes and which are non-limitative.